test: drive Korean and Vietnamese through the engine's own transstr

ko_test and vi_test replayed keys with private copies of transstr's
loop, and test_util reimplemented the shown preedit; the engine exports
impre(Im*, Str*) and transstr() instead, so a change to composition
rules is one edit.

strans.c

@@ -148,13 +148,13 @@ haspre(Im *p)
 }
 
 /* The preedit as clients see it: Telex keys read back through the map. */
-static void
-impre(Str *s)
+void
+impre(Im *p, Str *s)
 {
-	if(isjp(&im))
-		jpreading(im.l->map, &im.pre, &im.raw, s);
-	else if(!mapget(im.l->map, &im.pre, s))
-		*s = im.pre;
+	if(isjp(p))
+		jpreading(p->l->map, &p->pre, &p->raw, s);
+	else if(!mapget(p->l->map, &p->pre, s))
+		*s = p->pre;
 }
 
 static void
@@ -167,7 +167,7 @@ snapshot(Drawcmd *dc)
 	if(search.lang)
 		pre = search.text;
 	else
-		impre(&pre);
+		impre(&im, &pre);
 	if(!(activecap & Cclientpreedit))
 		dc->pre = pre;
 	first = pagefirst();
@@ -456,7 +456,7 @@ foldascii(Str *s)
 }
 
 /* What typing raw in language l would produce, composition included. */
-static void
+void
 transstr(Lang *l, Str *raw, Str *out)
 {
 	Im q;
@@ -827,7 +827,7 @@ imhandlekey(Keyreq *kr)
 		if(search.lang)
 			res.preedit = search.text;
 		else
-			impre(&res.preedit);
+			impre(&im, &res.preedit);
 	}
 	chansend(kr->reply, &res);
 }